Advancing Electrochemical Nitrate Reduction: Overcoming Rate‐Limiting Bottlenecks with Copper/Cobalt Catalysts

Abstract Electrochemical Nitrate Reduction (NO ₃ ⁻ RR) is a promising green process for producing ammonia and treating waste water. The nitrate‐to‐ammonia reduction involves multi‐step electron/proton‐transfer processes, where the NO ₃ ⁻ →NO ₂ ⁻ step may serve as the rate‐determining step, posing a critical bottleneck for efficient NH ₃ synthesis. In this paper, the emulsion hydrothermal method is used to synthesize spherical and nanoflower‐like CuO/CuCo ₂ O ₄ catalysts with small particle stacking. Among them, CuCo ₂ O ₄ perfectly inherits the advantages of CuO and Co ₃ O ₄ , and successfully connects the two‐step reactions of NO ₃ ⁻ →NO ₂ ⁻ and NO ₂ ⁻ →NH ₃ in series. The CuO formed by excess copper doping is reduced to monomeric copper during electrolysis. Cu is able to synergize with CuCo ₂ O ₄ to break through the bottleneck of the rate‐limiting step of NO ₃ ⁻ →NO ₂ ⁻ , exhibiting almost the same ammonia production efficiency in both NO ₃ ⁻ RR and nitrite reduction reaction (NO ₂ ⁻ RR). The NH ₃ yield of Cu/CuCo ₂ O ₄ at −0.70 V (vs RHE) reached a maximum of 24.58 mg h ⁻¹ mg _cat ⁻¹ under neutral electrolyte conditions and exhibited 100% Faraday efficiency for NH ₃ . Under the same conditions (where the reaction substrate is NO ₂ ⁻ ), Cu/CuCo ₂ O ₄ reached an NH ₃ yield of 24.34 mg h ⁻¹ mg _cat ⁻¹ in NO ₂ ⁻ RR.